This invention relates to a method of making a lug journal for a roller cutter drill bit, and more particularly to the method of finish machining such a lug journal by a cutting tool while the lug journal is mounted within a workpiece holding fixture on a stationary stand.
A rotary drill bit is normally formed of three generally identical elongated body sections or lugs which are welded together to form the completed drill bit. Each lug provides 120.degree. of the completed drill bit body. Heretofore, it has been common to mount the lug or workpiece within a workpiece holding fixture on a rotary fixture of a lathe and to rotate the fixture relative to a cutting tool positioned in a fixed holder outwardly of the fixture. The lug must be accurately positioned on the lathe in order for the lug journal to be machined properly and precisely. Also, in order to provide a balancing of the lathe with the workpiece mounted thereon, a suitable counterbalance has been mounted on the fixture in an opposed relation to the workpiece to provide balancing of the rotating fixture. Oftentimes, the counterbalance is not of the identical weight or configuration as the workpiece, and it is difficult also during setup to align the centerline of the semifinished lug journal with that of the cutting centerline of the rotating fixture closer than around 0.010 inch. These counterbalancing difficulties are usually not critical except in relatively large diameter drill bits over around twelve inches in diameter, for example. Thus, as a result of imbalance it is difficult to finish machine case hardened lug journals for a rock bit having a diameter over twelve (12) inches on a lathe rotating a fixture holding the lug. It has heretofore been necessary to finish grind these lug journals for large bits in order to achieve the desired dimension tolerances and surface finishes. Dimension tolerances less than about 0.005 inch and surface finishes less than 35 RMS must be maintained which results in a costly operation.